1. Field of the Invention
The invention relates to image display n particular to a method of image encoding by which pixel information for an image is encoded into digital data which can be recorded on or in a suitable storage device and to a method of decoding by which such digital data may be decoded to reconstitute the original pixel information.
The invention relates in particular to a method of image encoding which comprises the steps: a) receiving the pixel information in the form of a first matrix of m.times.n pixel component values where m and n are integers, b) decimation filtering the first matrix of pixel component values to produce a second matrix m/a.times.n/b pixel component values of reduced density where a and b are factors of m, c) encoding the second matrix of pixel component values into a first set of digital data for storage, by reference to a finite range of unsigned quantisation values, which range includes a guard range extending below black level, d) decoding the encoded second matrix of pixel component values to form a third matrix of m/a.times.n/b pixel component values, e) expanding the third matrix of pixel component values to form a fourth matrix of m.times.n pixel component values, f) forming the difference between the fourth and the first matrix pixel-by-pixel to produce a fifth matrix of m.times.n difference values, and g) encoding the fifth matrix of difference values into a second set of digital data for storage,
2. Description of the Related Art
Such a method is described in published European patent applications EP-0272762-A and EP-0272763-A and allows a high resolution image (for example 768.times.560 pixels) to be encoded for storage in an efficient manner by the encoding of a low resolution image (384.times.280 pixels) in the first set of data together with a high resolution difference image in the second set of data. Since the difference image contains only the high spatial frequency components of the image, it can be coded with a high degree of compression, so that the two sets of data occupy less space than would otherwise be required to store the high resolution image. The known method was developed in particular with a view to storage of the two sets of digital data in a compact Disc-Read Only Memory (CD-ROM) for use in a display apparatus constructed in accordance with standards which have been established for Compact Disc-Interactive (CD-I) players. The limited data rate when reading from a CD-JTOM disc would then be just adequate to allow derivation therefrom of a display of a 784.times.560 pixel image within a time delay acceptable to the average user.
The CD-I standards define a `base case` CD-I player as one providing the lowest level of compatibility. Unfortunately, the difference values of the fifth matrix can be positive or negative with equal probability, while the decoder for pixel values in the base case CD-I player is constructed for the generation of unsigned (positive) colour values only. The decoding in the base case CD-I player is based on CCIR recommendations for 8-bit (per component) picture coding, namely an unsigned value in the range zero to 255 with black level at 16 and peak white at 235. The guard ranges at 0-15 and 236-255 serve to prevent arithmetic overflow errors arising from overshoot and other small errors in the differential coding of images.
The coding method previously disclosed therefore required hardware in the display apparatus which is not present in a `base case` CD-I player. While it is always possible to define extensions to the standard player to produce players with enhanced functionality, this is contrary to the objectives of standardisation in the marketplace. Furthermore, for a disc to be compatible with both base case players and extended players would require storage thereon of the high resolution image in its uncompressed form side by side with the first and second sets of digital data--a very inefficient use of the finite storage capacity of the CD-ROM.